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A general view shows New York's mayor Bill de Blasio speaking during the second day of an international summit of Mayors on "Modern Slavery and Climate Change" on July 21, 2015 at the Vatican. Mayors from around the world have signed a declaration with Pope Francis stating that man-made climate change is real and humanity has a moral imperative to take action. At a two-day summit in the Vatican aimed at spurring efforts to fight global warming following the pope's environment encyclical denouncing the exploitation of the poor and the earth's resources, over 60 mayors pledged to do their part. AFP PHOTO / TIZIANA FABITIZIANA FABI/AFP/Getty Images